The present disclosure relates to methods and devices for comparing and standardizing the appearance of radiographic images. More specifically, the present disclosure relates to methods and devices for using a variable attenuation plate (VAP) to aid in the interpretation of radiographs and to facilitate the comparison of radiographic images taken on different occasions.
Radiographic imaging is a common diagnostic tool used routinely in the health care system and is vital in patient care throughout the world. X-rays are a form of high energy electromagnetic radiation which can be attenuated, absorbed or scattered to varying extents by all materials, including tissues found in the human body. In general, a radiographic image is obtained by directing a beam of X-rays toward the appropriate area or part of a patient's body. A detector (which can be a film-based cassette or a digital detector) placed on the opposite side of the region being examined is then ‘exposed’ by the X-rays that have passed through the body tissues. Some structures such as bone have a higher density and will allow little if any of the X-ray beam to reach the detector. Other less dense tissues such as air-filled lungs will only minimally attenuate the beam, allowing most of the X-rays to pass through and reach the detector. Soft tissues, including organs such as heart and liver, will show an attenuation between that of bone and that of air. The resulting radiographic image formed on the detector therefore shows variations in intensity of exposure depending on the density of the body structures and tissues through which the X-ray beam has passed. The image can then be analyzed and interpreted by radiologists or other physicians.
The quality of a radiographic image depends on many variables. For example, the energy of the photons in the X-ray beam (as controlled by the peak kilovoltage (kVp) setting of the X-ray generator) affects the ability of the beam to penetrate body tissues. If the energy of the beam is too high, there will not be a sufficient difference between the ability of various body tissues to attenuate the beam, and the contrast of the resulting image will be reduced. As well, the intensity or quantity of photons in the beam (as controlled by the mA (milliamperes) or mAs (mA·s, milliampere seconds) setting of the X-ray generator), along with the exposure time, will affect the intensity as well as the contrast of the resulting image. A more intense X-ray beam will need less exposure time to provide an image of sufficient intensity. Optimal settings for a good quality image, including kVp and mA or mAs settings, are generally chosen depending upon the type of tissue being visualized. In addition, the patient needs to be properly positioned in the X-ray beam and as still as possible. Care needs to be taken to avoid exposing regions of the body or materials such as clothing, catheters, etc. that will attenuate portions of the X-ray beam, resulting in a change in the exposure of the underlying detector and thus a change in appearance of the radiographic image. For example, the extent to which the abdomen or shoulder is included when taking a chest radiograph will affect the exposure of the image obtained. Radiographs are often over- or underexposed due to inadequate control of these variables.
With the advent of digital imaging and increasingly powerful computer programs, radiologists and other health providers can now routinely manipulate digital radiographic images manually using Picture Archiving and Communication Systems (PACS), so as to optimize differences in the attenuation and contrast between body structures, and to attempt to equalize images of a patient taken on different occasions so as to facilitate comparison. Because some body structures will be better visualized with ‘darker’, more contrasted images while others are better visualized with ‘lighter’, less contrasted images, the manual adjustment of the density and contrast of digital images, or “windowing”, is a very useful tool in the routine interpretation of radiographs. However, digital adjustment of images can lead to potential inconsistency in reporting because manipulation of the images is generally based on the personal preferences of individual interpreters. For example, two different radiologists interpreting the same image may window the radiographic image differently and thus come to different conclusions, and the image alterations made by one radiologist may not be available to the other for comparison or assessment. A recent study of 6 health care professionals interpreting chest radiographs has shown that there can be a 50% or higher rate of disagreement as to whether the disease process being examined was improving or deteriorating.
As an example, chest radiographs are routinely performed to diagnose fluid in the lungs (congestion). Normally, lung tissue weakly attenuates an X-ray beam because of the air contained within it, and therefore provides a relatively dark radiograph image. As the air is replaced by fluid in a more congested lung, the X-ray beam is more strongly attenuated, resulting in a ‘whiter’ radiographic appearance. It can be difficult for a radiologist interpreting such a lung radiograph to know whether the “whiteness” of the image is due to increased fluid or merely due to other variable factors as described above. The problem can be compounded if the radiographic image has also been digitally manipulated, or windowed, to facilitate interpretation, as described above.
In addition, radiographic images of the same subject may be taken on two or more occasions in order to monitor the progress of a particular medical treatment. For example, follow-up radiographs for assessing healing of broken bones or follow-up chest radiographs for patients with heart or lung conditions, such as congestive heart failure, or patients undergoing cancer treatment are common. In such cases, it can be difficult to tell whether differences in the appearance of the subsequent images are due to real, medically relevant changes in the patient's condition or simply due to variations in capturing and/or processing the radiographic image at each occasion, as described above.
There is therefore a need for a method and device that will allow radiographic images to be interpreted consistently in the face of variations in the conditions under which the image is obtained. In addition, there is a need for a method and device that facilitates efficient and reliable comparison of radiographic images taken on different occasions.